Moved contrib.csrf.* to core code.

There is stub code for backwards compatiblity with Django 1.1 imports.

The documentation has been updated, but has been left in
docs/contrib/csrf.txt for now, in order to avoid dead links to
documentation on the website.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11661 bcc190cf-cafb-0310-a4f2-bffc1f526a37